Zeeman effect in sulfur monoxide: a tool to probe magnetic fields in star forming regions

[Context] Magnetic fields play a fundamental role in star formation processes and the best method to evaluate their intensity is to measure the Zeeman effect of atomic and molecular lines. However, a direct measurement of the Zeeman spectral pattern from interstellar molecular species is challenging due to the high sensitivity and high spectral resolution required. So far, the Zeeman effect has been detected unambiguously in star forming regions for very few non-masing species, such as OH and CN.[Aims] We decided to investigate the suitability of sulfur monoxide (SO), which is one of the most abundant species in star forming regions, for probing the intensity of magnetic fields via the Zeeman effect.[Methods] We investigated the Zeeman effect for several rotational transitions of SO in the (sub-)mm spectral regions by using a frequency-modulated, computer-controlled spectrometer, and by applying a magnetic field parallel to the radiation propagation (i.e., perpendicular to the oscillating magnetic field of the radiation). To support the experimental determination of the g factors of SO, a systematic quantum-chemical investigation of these parameters for both SO and O has been carried out.[Results] An effective experimental-computational strategy for providing accurate g factors as well as for identifying the rotational transitions showing the strongest Zeeman effect has been presented. Revised g factors have been obtained from a large number of SO rotational transitions between 86 and 389 GHz. In particular, the rotational transitions showing the largest Zeeman shifts are: N,J = 2, 2-1, 1 (86.1 GHz), N,J = 4, 3-3, 2 (159.0 GHz), N,J = 1, 1-0, 1 (286.3 GHz), N,J = 2, 2-1, 2 (309.5 GHz), and N,J = 2, 1-1, 0 (329.4 GHz). Our investigation supports SO as a good candidate for probing magnetic fields in high-density star forming regions.This work has been supported in Bologna by MIUR “PRIN 2012, 2015” funds (project “STAR: Spectroscopic and computational Techniques for Astrophysical and atmospheric Research” – Grant Number 20129ZFHFE, project “STARS in the CAOS (Simulation Tools for Astrochemical Reactivity and Spectroscopy in the Cyberinfrastructure for Astrochemical Organic Species)” – Grant Number 2015F59J3R) and by the University of Bologna (RFO funds), in Mainz by the Deutsche Forschungsgemeinschaft (DFG 370/6-1 and 370/6-2). S.C. acknowledges support from the AIAS-COFUND program, grant agreement No. 609033. A.A.R. acknowledges financial support by the Spanish Ministry of Economy and Competitiveness through project AYA2014-60476-P and the Ramón y Cajal fellowships. J.C. thanks spanish MINECO for funding under grants AYA2012-32032, AYA2016-75066-C2-1-P, CSD2009-00038 (ASTROMOL) under the Consolider-Ingenio Program, and the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013)/ERC-SyG-2013 Grant Agreement No. 610256 NANOCOSMOS. The support of the COST CMTS-Actions CM1405 (MOLIM: MOLecules In Motion) and CM1401 (Our Astro-Chemical History) is acknowledged